Options for Improving Climate Modeling to Assist Water Utility ...

Water Utility Climate Alliance White Paper
Options for Improving Climate Modeling to Assist Water Utility Planning for Climate Change
3.2.7 Have downscaling techniques been evaluated and compared?
The framework for evaluating downscaling errors described in Section 3.2.5 is rarely applied in
full. Generally, the evaluations end with documentation of performance of the baseline climate
simulation, so that much of the evaluation boils down to a sensitivity analysis of how the results
from the baseline climate simulation vary with alternative downscaling design.
One reason for the limited evaluation of downscaling techniques is the project-centric approach
to developing these techniques. The lack of coordinated effort means common analytical
procedures have not been established; intercomparison of downscaling techniques is rare outside
of a particular project; improvements are made to models as they relate to the specific interests of
a scientific project, but not necessarily in response to broader user community needs; and
common data archive standards are not established. The lack of coordination also has limited the
interaction between model developers and potential users of model output, such as water utility
planners, creating disconnect between the information provided and the information needed.
The few evaluations of statistical versus dynamical downscaling have found statistical
downscaling techniques and RCMs have comparable skill overall (Wood et al., 2004; Schmidli
et al., 2006). The choice of downscaling technique usually is dictated more often by the needs of
the group using the downscaled climate projections than the results from coordinated, systematic
intercomparison. This choice has been based on such factors as the computational limitations
available for the study, the regional extent for which the study is concerned, choice of output
variables, or the constraints of the system into which the downscaled data are used.
Evaluations of RCMs versus RCMs are often guided by the priorities of a scientific project
concerned with a particular mechanistic process – for example, simulating the response of
regional hydrological cycle to climate change – rather than the need for water utility planning.
The most comprehensive RCM evaluation in the United States will be conducted by the
NARCCAP (http://narccap.ucar.edu/). NARCCAP is designed to evaluate multiple-decade RCM
simulations using the framework described in Section 3.2.5 without the alternative baseline
climate simulations. Results from NARCCAP are just now being reported. More extensive multidecade
RCM intercomparison projects have been supported in Europe (PRUDENCE,
http://prudence.dmi.dk/; and ENSEMBLES, http://ensembles-eu.metoffice.com/), and some
results from these projects may be applicable to downscaling efforts in the United States.
Experimental procedures for comparison of alternative baseline climate simulations from RCMs
has been established by the GEWEX Hydrometeorology Panel Transferability Working Group
(http://rcmlab.agron.iastate.edu/twg/) through an Inter-Continental Transferability Study (ICTS;
http://icts.gkss.de/index.html). GEWEX is a core project of the World Meteorological
Organization World Climate Research Program, and ICTS engages the international community
through GEWEX in an experiment that to date includes simulations from six RCMs. The study
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